Nuclear-Reactors

Nuclear Reactors: Principle, Construction, Fueling and Types of Reactors

Nuclear Reactors

Nuclear reactors are the heart of a nuclear power plant. They contain and control nuclear chain reactions at a constant rate that produce heat through a physical process called fission. Reactors produce nuclear energy for industrial purposes. The heat is used to make steam that spins a turbine to generate electrical power.

Principle

Fission reactions take place at regulated rates. The role of a reactor in a nuclear power station is the same as that of the furnace in a thermal power station. Heat energy is produced in a fission reaction. This energy is used to rotate a turbine. The turbine turns the generator which produces electrical energy.

Reactors-Principle

Construction of Nuclear Reactor

Nuclear Reactor includes the following parts:

Core

It is the main part of the nuclear power plant. Here fuel is kept in the form of cylindrical tubes.

Fuel

Uranium is the basic fuel. Generally, pellets of uranium oxide (UO2) are arranged in tubes to form fuel rods. The rods are arranged into fuel assemblies in the reactor core.

The fuel rods may be separately loaded into the reactor (into tubes located in the reactor) or might be arranged into fuel assemblies. Each assembly consists of 50 to 200 fuel rods. The majority of reactor cores are about 12 to 15 feet high or long, depending on the reactor type.

Reactors-fuel

Moderator

Utilized to decrease the neutrons to thermal energies. Often the very same material is used as a coolant. Typical products that can be used as moderators include water, heavy water, and graphite.

Water is frequently utilized since it is plentiful and economical. In general, the better a product is at “moderating” the neutrons, the lower the fuel needs to be enhanced. Heavy water is much better than water, however, it is also pricey to produce.

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Graphite likewise is much better than water, however, a few of the effects of direct exposure of graphite products to radiation are unwanted.

Absorbing or Control Rods

These are made with neutron-absorbing material such as cadmium, hafnium or boron, and are inserted or withdrawn from the core to control the rate of reaction, or to stop it.

In some reactors, unique control rods are utilized to allow the core to sustain a low level of power effectively.

Heat Transfer

When fission takes place in the core, heat is produced and the temperature is as high as 1200 ᵒC. Water is pumped into the reactor under high pressure to gather heat.

When this very hot water is traveled through the heat exchanger where water is converted into steam to run the turbine and produce electrical power with the help of a generator.

The temperature of the steam coming out of the turbine is 300 ᵒC. This is further cooled down. To cool this steam generally water from river or sea is utilized.

Pressure vessel or pressure tubes

Usually, a robust steel vessel containing the reactor core and moderator/coolant, but it may be a series of tubes holding the fuel and conveying the coolant through the surrounding moderator.

Containment

The structure around the reactor and associated steam generators are developed to protect it from outdoor intrusion and to safeguard that outside from the effects of radiation in case of any major breakdown inside. It is usually a meter-thick concrete and steel structure.

Fueling Nuclear Reactor

Many reactors need to be closed down for refueling so that the reactor vessel can be opened. In this case, refueling is at periods of 12, 18, or 24 months, when a quarter to a third of the fuel assemblies are replaced with fresh ones.

Disposal of Hazardous Waste

The nuclear fuel once used for charging the reactor works for a few months. The utilized fuel is then removed from the reactor and fresh fuel is fed into the reactor.

In the utilized fuel, there is a large number of radioactive materials. Their half-life is about thousands of years. The radiations from these used fuels are very hazardous and harmful for livings. Proper arrangements need to preserve nuclear disposal.

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Salt mines

The best place for the preservation is in the bottom of old salt mines which are countless meters below the surface area of Earth. Nuclear materials cannot be dumped into oceans or left anywhere as they will contaminate the environment through soil and water.

Salt-mines

Types of Reactors
Thermal Reactors

These are ones in which moderators are used to decreasing the quick neutrons to thermal energies so that they can produce more fission. Either natural uranium or enriched uranium is utilized as a fuel in these reactors. Enriched uranium is one in which the percentage of 23592 U is greater than its portion in natural uranium.

There are many designs of thermal reactors but the most commonly used are Pressurized Water Reactor (PWR).

Pressurized water reactor (PWR)

This is the most typical type, with about 300 operable reactors for power generation and a number of hundred more used for naval propulsion. The design of PWRs stemmed from a submarine power plant.

PWRs utilize ordinary water as both coolant and moderator. The style is identified by having a main cooling circuit that flows through the core of the reactor under really high pressure, and a secondary circuit in which steam is produced to drive the turbine.

Boiling water reactor (BWR)

This type of reactor has lots of similarities to the PWR, other than that there is just a single circuit in which the water is at lower pressure (about 75 times atmospheric pressure) so that it boils in the core at about 285 ° C.

The reactor is developed to run with 12-15% of the water in the top part of the core as steam, and for this reason with less moderating impact and hence efficiency there. BWR systems can run in a load-following mode more readily than PWRs.

Fast neutron Reactor

These are one in which natural uranium is used as a fuel which is almost 99% of Uranium. 23892U takes in a neutron and is converted into 239 plutonium.

Fast neutrons can produce fission, so moderators are not needed. A core of a fast reactor consists of a mixture of plutonium and uranium dioxide. The neutrons which get away from the core react with 23892 U in the blanket and finally produce 239- plutonium.

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Hence more plutonium fuel is bred therefore natural Uranium is utilized more effectively. These quick reactors are also called fast breeder reactors.